Mainly mojoPortal was designed with the idea to provide a business most of the functionality they would want for a web site. It was not designed to be like community server where a bunch of people have their own blog. See the section on our About page for What mojoPortal Was NOT Designed for.
Also we support the concept of multiple sites in a single installation where the first folder segment of the url identifies the site, therefore we kept other aspects of the url simple without extra segments.
I will log this as a feature request, but it is probably something that will not be worked on any time soon.